1. Field of the Invention
This invention generally relates to a flexible shaft which transmits a torque under its freely bent condition.
2. Discussion of Relevant Art
A flexible shaft is generally used for various purposes such as a portable mowing machine, industrial machines or the like.
In a general shoulder type mowing machine, an end portion of a pipe 2 is fixed to an engine 1, as illustrated by FIG. 2. A cutter blade 3 is supported to the other end of the pipe 2. Two grips 8 for allowing an operator to hold the mowing machine and a band 9 to be carried on an operator's shoulder are secured onto an outer periphery of the pipe 2. A flexible shaft 5 is inserted in and through the pipe 2, as illustrated in FIG. 3. The flexible shaft 5 is composed of an inner shaft 6 and a liner tube 7 which covers an outer periphery of the inner shaft 6. The inner shaft 6 is connected to an output shaft (not shown) of the engine 1 at one end there of and connected to the cutter blade 3 at another end thereof. Both ends of the liner tube 7 are secured firmly so that they do not rotate together with the inner shaft 6. Accordingly, a torque of the output shaft of the engine 1 is transmitted to the cutter blade 3 through a clutch 4 and the inner shaft 6 of the flexible shaft 5. The liner tube 7 is manufactured in such a manner that an outer periphery of a spiral tube 11, which is formed of closely wound element wires made of steel, is covered by a braid 12; and an outer periphery of the braid 12 is covered by a coating 13 of rubber and vinyl etc.; as illustrated by FIG. 4. The coating 13 may be formed of an extruded liner made of synthetic resin.
In a back type mowing machine, the pipe 2 is installed only between the grip 8 and the cutter blade 3; and the flexible shaft 5 is exposed in a range between the engine 1 and the grip 8, as illustrated in FIG. 5 Consequently, the flexible shaft can be freely bent within the range between the engine 1 and the grip 8.
Generally, the inner shaft 6 is manufactured such that element wire groups, which are formed by closely adhering multiple steel element wires into band shapes, are wound around a steel core wire by turns in reverse directions and a plurality of layers of element wire groups having concentric sections are formed thereon. In assembling the flexible shaft 5, grease of about 10 grams, for example, is filled in the liner tube 7 as lubricant; and the inner shaft 6, an outer periphery of which is applied with grease of about 10 grams, for example, is inserted in the liner tube 7. In carrying out mowing work, the inner shaft 6 rotates in the fixed liner tube 7 at a rotation speed of about 7,000 to 8,000 rpm. However, since another end 2a of the pipe 2 is bent downward so that the cutter blade 3 is positioned in approximately parallel position with the ground surface under a condition where the operator holds the pipe 2, heat generation of the flexible shaft 5 increases in the other end 2a; and consequently, grease deteriorates. For this reason, it has been required for users to take out the inner shaft 6 from the liner tube 7 after every 50 operation hours, for example, to remove old grease, and insert the inner shaft 6 into the liner tube 7 while applying new grease onto the outer periphery of the inner shaft 6. Further, the conventional flexible shaft 5 exhibits a defect in durability because of its wear and tear which is inevitable due to wear of the inner shaft 6; notwithstanding the fact that the grease has been periodically replaced.
More particularly, in the back type mowing machine of FIG. 5, the above-mentioned problem becomes remarkable because the exposed part of the flexible shaft 5 is repeatedly and frequently bent.